1. Field of the Invention
The present invention relates to wavelength division multiplexed (WDM) optical signal amplifiers, and more particularly, to a WDM optical signal amplifier that maintains its optical output power at a constant level.
2. Description of the Related Art
As a way of amplifying optical transmission signals, optical amplifiers have become a reality in recent years, which operate entirely in the optical domain and do not employ any translation of data back to electrical baseband signals. Optical amplifiers include optical fiber amplifiers which boost optical signals by using a special optical fiber doped with rare-earth elements such as Erbium. Semiconductor laser amplifiers having a semiconductor-laser-based structure are another type of optical amplifiers, which are pumped by an appropriate amount of injection current set below the threshold point.
In general, optical amplifiers can be used for the amplification of WDM optical signals. In other words, they can amplify, at a time, two or more optical signals each having a different wavelength.
Optical amplifiers often employ an output feedback control mechanism to regulate their optical output at a constant power level. In optical fiber amplifiers, this feedback control is accomplished by varying the power of pumping light beams according to the actual output power level. In semiconductor laser amplifiers, on the other hand, the injection current is manipulated to control the output power level. Such feedback control mechanisms successfully provide a well-regulated optical signal output when the signal being amplified contains only a single wavelength component.
However, when the signal consists of two or more optical components each having a different wavelength, the above-described feedback control is of no help in reducing the variation in power levels among the different wavelength components contained in the optical output signal. The amplifier may seem to keep its output at a constant power level. In reality, however, it is only regulating the total output power level, but not the levels of individual signal components contained in the optical output signal, and therefore, the optical signal components with different wavelengths would exhibit different signal strengths. It is noted here that the optical amplifiers are required to have a flat response to different wavelengths.